Electrostatic precipitators are well known and commonly utilized especially in controlling particulate emissions. Other forms of electrical discharge devices for the same or similar purposes are less well known but available. For example, there are corona cells, pulsed corona cells, silent barrier discharge devices and surface discharge devices which are also receiving attention.
Generally, in electrostatic precipitators, a high voltage is applied to electrodes to produce a corona discharge. Dust or droplets in the gas flowing through the device are charged by electrons and ions of the corona discharge and they are then attracted to and collected by an oppositely charged electrode. It is known that the collection efficiency of these devices can be increased if beam-like electrons (electrons having energies higher than about 50 electron volts) are used rather than lower energy thermal-like electrons (electrons having energies typically less than 10 electron volts). In fact, thermal electrons in precipitation chambers typically have energies as low as one electron volt.
Thermal electrons are produced about 50 nanoseconds after a high-voltage pulse is applied and the higher energy beam-like electrons are dissipated as soon as the thermalized electrical discharge takes place. It would be desirable of course to produce beam-like electrons rather than thermal-like electrons to achieve highest efficiency.
Another factor affecting efficiency of operation is the limitation of current which occurs when voltage is limited to avoid sparking.
Conventional pulsed corona reactors for controlling gaseous pollutants and particulates are usually cylindrical chambers having a thin (0.1-3.0 mm.) wire axial high-voltage electrode. Gas to be treated flows through the chamber. Pulsed streamers initiated at the wire propagate towards the chamber wall and cause reduction and decomposition of pollutants in the gas. With limited voltage, the radial electric field rapidly decreases, and the streamers' propagation and development is arrested. As a result, most of the energetic electrons are generated in close proximity to the wire. This "effective plasma volume" is localized within a volume that is only the width of a few wire radii. This limitation decreases the efficiency of corona reactors and raises the energy cost of effluent gas treatment.
Therefore, it is an object of the present invention to increase the efficiency of a corona reactor by increasing the ratio of beam-like electrons to thermal-like electrons.
Another object of the invention is to provide a structure in which streamer discharge is initiated from both electrodes of a reactor, thereby increasing "effective plasma volume".
Yet another object is to provide input pulses to a corona reactor of a repetition rate, a pulse width, and amplitude to maximize efficiency of operation of a reactor chamber in reducing pollutants such as nitrous oxide, hydrocarbons and particulates.